1. Field of the Invention
The present invention relates to laser machining and more specifically, it relates to improving cut quality and rate by appropriately orienting the polarization vector of a series of ultrashort laser pulses when laser machining materials.
2. Description of Related Art
Lasers have been used to machine or cut a target comprised of a rigid material, such as metals, wood, rubber, plastics or ceramics. In the machining process, a laser beam thermally reacts with the target and vaporizes the material to remove material from the target. Conventionally, the beam output from such a laser is focused onto the target material. Minimal significance has been given to the polarization of the beam used in laser machining processes. The state of polarization of the beam was thought to make very little difference in the size or shape of the aperture which resulted from the beam thermally reacting with the material. Where polarization has been observed to affect the quality of the cut, the prior art has taught that the use of conventional laser parameters produces an asymmetric cut. In addition, the depth of penetration is taught to be deeper, for conventional laser parameters, where the direction of polarization is parallel to the machining direction.
U.S. Pat. No. 4,336,439, issued Jun. 22, 1982, titled "Method and Apparatus for Laser Scribing and Cutting" by Sasnett et al., describes polarization control to remove material such that the removed portion is symmetrically shaped. The patent describes a laser system adapted for generating a high power laser beam which is aligned to impinge a target of a rigid material to thermally react with the material to remove a portion therefrom, and where a laser means for generating the high power beam of electromagnetic coherent radiation has a state of polarization. The beam is aligned to impinge the material. Controlling means are provided to control the polarization of the beam with respect to the material such that the portion removed is symmetrically shaped. In one embodiment, the incident beam is linearly polarized and the direction of polarization is perpendicular to the direction of machining. The resultant aperture produces an asymmetric hole which is curved back to the bottom in the direction opposite that of the direction of machining. In addition, the depth of penetration is not as deep, for the parameters disclosed, as another shown embodiment where the direction of polarization is parallel to the direction of travel. The disclosed laser parameters include use of a series of laser pulses of about 100-300 microsecond duration, with the beam (or the material) moving at a speed of about 10 inches per second and a pulse repetition rate of between 1500 and 2000 pulses per second.
U.S. Pat. No. 4,547,651, issued Oct. 15, 1985, titled "Laser Machining Apparatus" provides a laser machining apparatus comprising an optical resonator including a pair of partially and totally reflecting mirrors disposed in opposite relationship to each other and a laser medium disposed between the pair of the partially and totally reflecting mirrors to generate a laser beam through the amplification by the laser medium, at least one polarizer disposed between the partially and totally reflecting mirrors to cross the laser beam and to be rotatable about the optical axis of the laser beam, the polarizer linearly polarizing the laser beam, and a driving means for rotating the at least one polarizer about the optical axis of the laser beam so that the linearly polarized laser beam has a plane of polarization coinciding with a direction in which the linearly polarized laser beam machines a workpiece.
U.S. Pat. No. 5,720,894, issued Feb. 24, 1998, titled "Ultrashort Pulse High Repetition Rate Laser System for Biological Tissue Processing" describes systems for removal of biological and other types of material with minimal collateral damage and greatly increased cut quality by using laser pulses of duration less than 100 picoseconds. More specifically, the duration of each laser pulse is on the order of about 1 fs to less than 50 ps such that energy deposition is localized in a small depth and occurs before significant hydrodynamic motion and thermal conduction, leading to collateral damage, can take place. The depth of material removed per pulse is on the order of about 1 micrometer, and the minimal thermal and mechanical effects associated with this ablation method allows for high repetition rate operation, in the region 10 to over 1000 Hertz, which, in turn, achieves high material removal rates. The input laser energy per ablated volume of tissue is small, and the energy density required to ablate material decreases with decreasing pulse width. The ablation threshold and ablation rate are only weakly dependent on material type and condition, allowing for maximum flexibility of use in various material removal applications. The use of a chirped-pulse amplified Titanium-doped sapphire laser is disclosed as the source in one embodiment.